1. Field of the Invention
The present invention relates to an apparatus, such as a projection exposure apparatus, which requires a focusing operation with respect to a substrate (photosensitive substrate) to be processed and, more particularly, to a technique of performing a focusing operation reliably for a substrate to be processed on which the undulations of a processing area are uneven.
2. Related Background Art
Conventionally, some projection exposure apparatuses for exposing the pattern of an integrated circuit onto the surface of a semiconductor wafer or the like adopt an optical focus position shift detecting method in order to match the wafer surface accurately with a predetermined exposure plane. As one type of this optical focus position shift detecting method, there has been provided a method of obliquely radiating a light beam with an elongated section downward onto a wafer as a surface to be detected, converging reflected light components of the light beam through a vibrating slit, and detecting a focus position by using an output obtained by photoelectrically converting the converged light signal and electrically processing the converted signal. The detecting light beam used in this focus position shift detection, however, is constantly radiated onto a predetermined position of a wafer at rest. Therefore, although a focusing operation can be performed on a local plane at an irradiated position, if a surface with a height different from that of the plane at the position irradiated with the detecting light beam is present within a projection exposure area (field), this surface does not agree with the focal plane. In this case, an inconvenient situation results if the design rule of the pattern of an integrated circuit on the surface at the level different from that of the plane at the irradiation position of the detecting light beam is severest in the projection exposure area and an in-focus state must be obtained on that surface.
Recently, the development of high-NA projection exposure lenses has advanced with a reduction in the line widths of IC patterns, and this consequently has decreased the margin for the depth of focus. If, therefore, the levels of individual circuit patterns within a projection exposure area are uneven, it is desirable that a focusing operation be performed in accordance with a circuit pattern requiring a minimum focus position shift, among other circuit patterns.
For this reason, in conventional apparatuses, the difference in level between a plane at a position irradiated with a detecting light beam used in focus position shift detection and a plane at a position of a certain circuit pattern requiring a minimum focus position shift is designated beforehand as a focus offset amount. When a focal point is to be actually matched with the plane at the irradiation position of the detecting light beam, the focus offset amount designated beforehand is corrected by electrical processing. In this method, however, the focus offset amount must be measured in advance.
It is also possible to perform a method (focus lock) in which a focusing operation is performed by moving the irradiation position of a detecting light beam used in detection of a focus position shift to the position of a circuit pattern requiring a minimum focus position shift, and then exposure is performed by moving the irradiation position to a projection exposure position while the obtained focal point is maintained. This method, however, has a drawback that the processing time increases each time the exposure processing is performed.
Even when, on the other hand, an alignment sensor performs an alignment operation with respect to an alignment mark on a substrate to be processed, a position irradiated with a detecting light beam used in focus position shift detection is generally different from the position of the alignment mark. Therefore, similar problems arise if the level of a plane at the irradiation position of the detecting light beam is different from that of a plane at the position of the alignment mark.